PROJECT SUMMARY The BRCA1 and PALB2 proteins directly heterodimerize through their respective coiled-coil (CC) domains, facilitating the formation of a larger BRCA1-PALB2-BRCA2-RAD51 complex that is required for RAD51 filament formation. Currently, little is known about the structural basis of the BRCA1-PALB2 interaction, including the residue alignment and orientation of physically interacting ?-helices. Elucidating CC molecular interactions is critical for understanding how mutations found in patients disrupt peptide interactions and promote disease. Protein CC domains are capable of mediating interactions with several CC domain containing partners. However, BRCA1 and PALB2 form the only known interaction that is mediated by their respective CC domains. Whether BRCA1 and PALB2 CC domains exclusively interact with one another, or if there are additional CC interactions and functions is unknown. Our laboratory has developed novel BRCA1 and PALB2 CC domain mutant mouse models to investigate the significance of this interaction in DNA repair and organismal health. We have also purified CC peptides so that biochemical assays can be performed assessing the effects of mutations on complex interactions and activity. The only known function of BRCA1 and PALB2 CC domains is to interact with one another, thus, BRCA1CC and PALB2CC homozygous mice might be expected to have identical phenotypes. However, while BRCA1CC mice are born at sub-Mendelian ratios and neo-natal mice demonstrate a range of developmental defects, PALB2CC homozygosity resulted in early embryonic lethality. Because BRCA1CC and PALB2CC mice have distinct phenotypes, we hypothesize that CC domains facilitate protein interactions beyond the BRCA1-PALB2 heterodimer that promote DNA repair and embryonic development. We will address the following Specific Aims: 1) Determine biochemical CC interactions and activity; 2) Uncover DNA repair and developmental defects in CC mutant mice; and 3) Elucidate mechanisms of BRCA1-PALB2 complex recruitment to DNA breaks. Collectively, the proposed experiments will yield new insight into the mechanism by which the BRCA1-PALB2 complex protects from genome instability.